Automatic frequency control device and mehtod of QPSK modulation system

ABSTRACT

An AFC (Automatic Frequency Control) device of a QPSK modulation system including a frequency discriminating unit for calculating a real part peak value using frequency discrimination between adjacent I and Q signals of a received sequence and an imaginary part peak value using the frequency discrimination between adjacent I signals and adjacent Q signals, an error detecting unit for detecting an error symbol from the received sequence, and a switching unit for removing the detected error symbol under the control of the error detector. Also included is a frequency difference average calculating unit for calculating a frequency difference of the received sequence without the error symbol.

CROSS-REFERENCE TO A RELATED APPLICATION

This application claims priority to Korean application No. 56523/2003filed Aug. 14, 2003, the entire contents of which are herebyincorporated in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an AFC (Automatic Frequency Control) ofa QPSK (Quadrature Phase Shift Keying) modulation system, and moreparticularly to an AFC device and method of a QPSK modulation systemcapable of raising an accuracy of the AFC by calculating a frequencydifference excluding an error symbol.

2. Background of the Related Art

In general, when a channel is changed in a mobile communication system,a transmission/reception frequency is also changed. Thus, a receivingside needs to be tuned with the corresponding changed frequency used bythe sending side.

Further, the receiving side uses an AFC device to detect an allocatedchannel frequency. In addition, the receiving side also uses the AFCdevice to prevent degradation of the receive sensitivity due to amulti-path fading phenomenon.

A general AFC device calculates frequency differences between adjacent Iand Q signals using a frequency discriminator of a cross product type,calculates an average value of the calculated frequency differences, andthen performs AFC using the obtained average value.

In this manner, the general AFC device calculates the frequencydifference using a symbol of a received training sequence and performsAFC using the calculated frequency difference. However, the general AFCdevice calculates a frequency difference of the training sequenceincluding an error symbol, which reduces the accuracy of the frequencydifference value and degrades the performance of the AFC device.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to address at leastthe above and other noted objects.

Another object of the present invention is to remove influences of theerror symbol on the frequency difference values and raise the accuracyof AFC.

To achieve at least the above objects in whole or in parts, the presentinvention provides a novel AFC device of a QPSK modulation systemincluding a frequency discriminating unit for calculating a real partpeak value using a frequency discrimination between adjacent I and Qsignals in a received sequence and an imaginary part peak value using afrequency discrimination between adjacent I signals and adjacent Qsignals in the received sequence, and an error detecting unit fordetecting an error symbol from the received sequence. The device alsoincludes a switching unit for removing the detected error symbol underthe control of the error detecting unit, and a frequency differenceaverage calculating unit for calculating an average of frequencydifference values of the received sequence without the error symbol. Thepresent invention also provides a novel AFC method.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIG. 1 is a block diagram of an AFC device of a QPSK modulation systemaccording to the present invention;

FIG. 2 is an overview of a double product type frequency discriminatingunit;

FIG. 3 is an overview of a frequency difference average calculatingunit;

FIG. 4 is a flowchart of an AFC method of a QPSK modulation systemaccording to the present invention;

FIG. 5 illustrates real and imaginary part peak values output betweenadjacent symbols by the double product type frequency discriminatingunit;

FIG. 6 is a graph showing a relation among the real peak value, theimaginary part peak value and a frequency difference between adjacentsymbols; and

FIG. 7 is a graph illustrating an error symbol decision region.

BEST MODE OF THE INVENTION

In the present invention, an AFC device calculates a real part peakvalue using frequency discrimination between I and Q signals of adjacentsymbols and an imaginary part peak value using frequency discriminationbetween adjacent I signals and adjacent Q signals using a double producttype frequency discriminating unit, calculates a frequency difference ofinput symbols using the calculated two types of peak values, andperforms an AFC using the calculated frequency difference.

In addition, according to the present invention, a received symbol,which does not fall within a decision region, is detected, and then afrequency difference is calculated excluding the detected symbol.

Turning now to FIG. 1, which is a block diagram of an AFC device of aQPSK modulation system according to the present invention. As shown, anAFC device of a QPSK modulation system includes a double product typefrequency discriminating unit 100 for receiving a QPSK type inputsequence, sequentially calculating a real part peak value using theproduct of adjacent I and Q signals, and sequentially calculating animaginary part peak value by using the product of adjacent I signals andthe product of adjacent Q signals; an error detecting unit 200 fordetecting an error symbol using the real and imaginary part peak valuesand outputting a control signal to remove a real part peak value and animaginary part peak value according to the detected error symbol; aswitching unit 300 for selectively bypassing the real and imaginary partpeak values outputted from the double product type frequencydiscriminating unit 100 according to the control signal; and a frequencydifference average calculating unit 400 for calculating an average ofthe real part peak values and an average of the imaginary part peakvalues outputted from the switching unit 300 and calculating a frequencydifference average of the inputted sequence using the calculatedaverages of the real part peak value and the imaginary part peak value.

With reference to FIG. 2, the double product type frequencydiscriminating unit 100 includes first single type frequencydiscriminators for calculating a real part peak value using the productof adjacent I and Q signals; and second single type frequencydiscriminators for calculating an imaginary part peak value using theproduct of adjacent I signals and the product of adjacent Q signals.

The first single type frequency discriminators include a first delayunit 110 for delaying an inputted I signal for a certain amount of time;a first multiplier 120 for multiplying the I signal delayed in the firstdelay unit 110 and an inputted Q signal; a second delay unit 130 fordelaying the inputted Q signal for a certain amount of time; a secondmultiplier 140 for multiplying the Q signal delayed in the second delayunit 130 and the inputted I signal; and a first adder 150 forcalculating a difference value between outputs of the first and secondmultipliers 120 and 140 and outputting a real part peak value(r_(c)(t)).

The second single type frequency discriminators include a thirdmultiplier 160 for multiplying the inputted I signal and the I signalwhich has been delayed for a certain amount of time; a fourth multiplier170 for multiplying the inputted Q signal and the Q signal which hasbeen delayed for a certain amount of time; and a second adder 180 forcalculating outputs of the first and second multipliers 160 and 170 andoutputting an imaginary part peak value (r_(s)(t)). The certain amountof time indicates a symbol period.

With reference to FIG. 3, the frequency difference average calculatingunit 400 includes peak average calculating units for calculating a realand imaginary part peak value averages of the inputted sequence using avector sum method; and a frequency difference output unit (not shown)for calculating a frequency difference average (Δf_(off)) of theinputted sequence using the calculated real and imaginary part peakvalue averages.

The peak average calculating units includes first and second adders 410and 420 for respectively adding the real and imaginary part peak valuesoutputted from the switching unit 300 to the real and imaginary partpeak value averages using the vector sum method; and a first averagecalculator 430 for outputting the real part peak value average using avalue outputted from the first adder 410 and providing the real partpeak value average to the first adder 410; and a second averagecalculator 440 for outputting the imaginary part peak value averageusing a value outputted form the second adder 420 and providing theimaginary part peak value average to the second adder 420.

The operation of the AFC device of the above-noted QPSK modulationsystem will now be described.

FIG. 4 is a flowchart of an AFC method of a QPSK modulation systemaccording to the present invention. In performing AFC, the trainingsequence known by both the sending and receiving side is used. Forexample, in a TD-SCDMA (Time Division Synchronous Code Division MultipleAccess) system, a training sequence of 144 chips is used.

When the training sequence of 144 chips is inputted in a QPSI complexsymbol form, the double product type frequency discriminating unit 100calculates the real part peak value (r_(c)(t)) and the imaginary partpeak value (r_(s)(t)) of adjacent symbols, and such calculation issequentially performed during the training sequence region (steps S10and S12). Thus, for example, if an input sequence is 144-chip trainingsequence, as shown in FIG. 5, the real part peak value (r_(c)(t)) andthe imaginary part peak value (r_(s)(t)) outputted from the doubleproduct type frequency discriminating unit 100 are 143, respectively.

The calculation of the real part peak value (r_(c)(t)) using thefrequency discrimination between adjacent symbols is as follows. When anI signal (I(t)) and a Q signal (Q(t)) of a QPSI complex symbol areinputted, the first multiplier 120 of the double product type frequencydiscriminating unit 100 multiplies an I signal (I signal of a previoussymbol) (I(t+T)) which has been delayed as long as a symbol period (T)through the first delay unit 110 and a currently inputted Q signal.Further, the second multiplier 140 multiplies a Q signal (Q signal ofthe previous symbol) (Q(t+T)) which has been delayed as long as a symbolperiod (T) through the second delay unit 130 and a currently inputted Isignal (I(t)). The first adder 150 calculates a difference between anoutput of the first multiplier 120 and an output of the secondmultiplier 140 and outputs a real part peak value (r_(c)(t)) between thecurrent symbol and the previous symbol.

In addition, the calculation of the imaginary part peak value (r_(s)(t))using frequency discrimination between adjacent symbols is as follows.The third multiplier 160 of the double product type frequencydiscriminating unit 100 multiplies a currently inputted I signal (I(t))and an I signal (I signal of a previous symbol)(I(t+T)) which has beendelayed as long as the symbol period (T) by the first delay unit (110).The fourth multiplier 170 multiplies a currently inputted Q signal(Q(t)) and a Q signal (Q signal of the previous symbol)(Q(t+T)) whichhas been delayed as long as the symbol period m by the second delay unit130. The second adder 180 adds an output of the third multiplier 160 andan output of the fourth multiplier 170, and outputs an imaginary partpeak value (r_(s)(t)) between the current symbol and the previoussymbol.

For a QPSK modulation system, if an input frequency difference is Δf,the input signals I(t) and Q(t) can be expressed by equations (1) and(2) shown below:I(t)=sin(2πΔft+θ)  (1)Q(t)=cos(2πΔft+θ)  (2)where θ indicates a phase of an input signal.

When the input signals I(t) and Q(t) are inputted, the real part peakvalue (r_(c)(t)) and the imaginary part peak value (r_(s)(t)) outputtedby the double product type frequency discriminating unit 100 can beexpressed by equations (3) and (4) shown below:r _(c)(t)=Q(t)×I(t+T)−I(t)×Q(t+T)=sin(2πΔft)  (3)r _(s)(t)=I(t)×I(t+T)+Q(t)×Q(t+T)=cos(2πΔft)  (4)where ‘T’ indicates a symbol period.

Further, a frequency difference can be calculated using an arbitraryreal part peak value (r_(c)(t)) and imaginary part peak value (r_(s)(t))by equation (5) shown below: $\begin{matrix}{{\Delta\quad f} = {\frac{1}{2\pi\quad T} \cdot {\tan^{- 1}\left( \frac{r_{c}(t)}{r_{s}(t)} \right)}}} & (5)\end{matrix}$

Next, FIG. 6 is a graph showing a relation among the real peak value,the imaginary part peak value and a frequency difference betweenadjacent symbols.

With reference to FIGS. 5 and 6, for a 144-chip training sequence, 143real part peak values (r_(c)(t)) and 143 imaginary part peak values(r_(s)(t)) can be calculated by the double product type frequencydiscriminating unit 100, and 143 frequency differences (Δf) betweensymbols are obtained using the 143 real part peak value (r_(c)(t)) andthe 143 imaginary part peak values (r_(s)(t)).

When 143 real part peak values (r_(c)(t)) and 143 imaginary part peakvalues (r_(s)(t)) for the 144-chip training sequence are sequentiallyoutputted at every symbol period by the double product type frequencydiscriminating unit 100, the error detecting unit 200 detects an errorsymbol based on the decision region of each QPSK symbol as shown in FIG.7 (step S14). The decision region is for deciding whether a receivedsymbol is normal. Accordingly, a symbol that does not fall within acorresponding region is regarded as an error symbol.

If a real part peak value (r_(c)(t)) and an imaginary part peak value(r_(s)(t)) calculated using a received specific symbol is within a rangeof equation (6) shown below, the specific symbol is regarded of beingout of the decision region. $\begin{matrix}{{{{Tan}\left( \frac{r_{c}(t)}{r_{s}(t)} \right)} > \frac{\pi}{4}},{{\tan\left( \frac{r_{c}(t)}{r_{s}(t)} \right)} < {- \left( \frac{\pi}{4} \right)}}} & (6)\end{matrix}$

Equation (6) can also be expressed by equation (7) shown below:$\begin{matrix}{{\frac{r_{c}(t)}{r_{s}(t)} > 1},{\frac{r_{c}(t)}{r_{s}(t)} < {- 1}}} & (7)\end{matrix}$

Accordingly, the error detector 200 can detect an error symbol accordingto whether or not the real part peak value (r_(c)(t)) and the imaginarypart peak value (r_(s)(t)) outputted from the double product typefrequency discriminating unit 100 satisfies the condition of equation(7).

If the real part peak value (r_(c)(t)) and the imaginary part peak value(r_(s)(t)) outputted from the double product type frequencydiscriminating unit 100 satisfies the condition of equation (7) (stepS16), the error detecting unit 200 determines that the real part peakvalue (r_(c)(t)) and the imaginary part peak value (r_(s)(t)) has beencalculated using an error symbol and controls the switching unit 300 sothat the real part peak value (r_(c)(t)) and the imaginary part peakvalue (r_(s)(t)) may not be bypassed. The switching unit 300 isxswitched off under the control of the error detecting unit 200 andremoves the real part peak value (r_(c)(t)) and the imaginary part peakvalue (r_(s)(t)) (step S18).

However, if the real part peak value (r_(c)(t)) and the imaginary partpeak value (r_(s)(t)) outputted from the double product type frequencydiscriminating unit 100 does not satisfy the condition of equation (7),the error detector 200 determines that the real part peak value(r_(c)(t)) and the imaginary part peak value (r_(s)(t)) have beencalculated by using a normal symbol and controls the switching unit 300so that the real part peak value (r_(c)(t)) and the imaginary part peakvalue (r_(s)(t)) can be bypassed.

The switching unit 300 is switched on under the control of the errordetecting unit 200 and bypasses the real part peak value (r_(c)(t)) andthe imaginary part peak value (r_(s)(t)) to the frequency differenceaverage calculating unit 400.

Then, the frequency difference average calculating unit 400 adds thereal part peak values (r_(c)(t)) and the imaginary part peak values(r_(s)(t)) outputted from the switching unit 300 in a vector sum methodto calculate a real part peak value average (R_(c)(t)) and an imaginarypart peak value average (R_(s)(t)) (step S20).

The frequency difference average calculating unit 400 also calculates afrequency difference average (Δf_(off)) of the training sequence fromthe obtained real part peak value average (R_(c)(t)) and the imaginarypart peak value average (R_(s)(t)) using the same method as equation (8)shown below (step S22): $\begin{matrix}{\left( {\Delta\quad f_{off}} \right) = {\tan^{- 1}\left( \frac{R_{c}(t)}{R_{s}(t)} \right)}} & (8)\end{matrix}$

The AFC device performs AFC using the frequency difference average(Δf_(off)) of the training sequence outputted from the frequencydifference average calculating unit 400 (step S24). In calculating thefrequency difference average (Δf_(off)) of the training sequence in theAFC device, a frequency difference calculated using an error symbol isexcluded to minimize an influence by the error symbol in AFC.

As so far described, the AFC device and method of a QPSK modulationsystem in accordance with the present invention have many advantages.

That is, for example, first, if the real and imaginary part peak valuesusing the frequency discrimination of adjacent symbols do not comewithin a certain range, a symbol used in calculating the real andimaginary part peak values is determined as an error symbol, therebysimply detect an error symbol of an input sequence.

Second, an error symbol is detected from an inputted sequence and afrequency difference of input symbols excluding the error symbol iscalculated, whereby an influence of the error symbol on the frequencydifference value is removed and thus the accuracy of the AFC isimproved.

Third, a real part peak value between an adjacent I signal and Q signaland an imaginary part peak value is calculated using the product ofadjacent I signals and the product of adjacent Q signals, and then afrequency difference is calculated using two types of peak values,thereby obtaining a more accurate frequency difference value.

This invention may be conveniently implemented using a conventionalgeneral purpose digital computer or microprocessor programmed accordingto the teachings of the present specification, as well be apparent tothose skilled in the computer art. Appropriate software coding canreadily be prepared by skilled programmers based on the teachings of thepresent disclosure, as will be apparent to those skilled in the softwareare. The invention may also be implemented by the preparation ofapplication specific integrated circuits or by interconnecting anappropriate network of conventional component circuits, as will bereadily apparent to those skilled in the art.

The present invention includes a computer program product which is astorage medium including instructions which can be used to program acomputer to perform a process of the invention. The storage medium caninclude, but is not limited to, any type of disk including floppy disks,optical discs, CD-ROMs, and magneto-optical disks, ROMs, RAMs, EPROMs,EEPROMs, magnetic or optical cards, or any type of media suitable forstoring electronic instructions.

The foregoing embodiments and advantages are merely exemplary and arenot to be construed as limiting the present invention. The presentteaching can be readily applied to other types of apparatuses. Thedescription of the present invention is intended to be illustrative, andnot to limit the scope of the claims. Many alternatives, modifications,and variations will be apparent to those skilled in the art.

1. An AFC (Automatic Frequency Control) device of a QPSK (QuadraturePhase Shift Keying) modulation system comprising: a frequencydiscriminating unit for calculating a real part peak value using afrequency discrimination between adjacent I and Q signals in a receivedsequence and an imaginary part peak value using a frequencydiscrimination between adjacent I signals and adjacent Q signals in thereceived sequence; an error detecting unit for detecting an error symbolfrom the received sequence; a switching unit for removing the detectederror symbol under the control of the error detecting unit; and afrequency difference average calculating unit for calculating an averageof frequency difference values of the received sequence without theerror symbol.
 2. The device of claim 1, wherein the real part peak valueis a difference value between the product of a Q signal of a currentsymbol and an I signal of a previous symbol and the product of an Isignal of the current symbol and a Q signal of the previous symbol. 3.The device of claim 1, wherein the imaginary part peak value is a valueobtained by adding the product of an I signal of a current symbol and anI signal of a previous symbol and the product of a Q signal of thecurrent symbol and a Q signal of the previous symbol.
 4. The device ofclaim 1, wherein the real part peak value and the imaginary part peakvalue are calculated at every symbol period.
 5. The device of claim 1,wherein a frequency difference between adjacent symbols is obtainedusing the following equation:$\frac{1}{2\pi\quad T} \cdot {\tan^{- 1}\left( \frac{r_{c}(t)}{r_{s}(t)} \right)}$wherein r_(c)(t) is the real part peak value, r_(s)(t) is the imaginarypart peak value, and ‘T’ indicates a symbol period.
 6. The device ofclaim 1, wherein the frequency discriminating unit comprises: a firstsingle type frequency discriminator for calculating the real part peakvalue using the product of adjacent I and Q signals; and a second singletype frequency discriminator for calculating the imaginary part peakvalue using the product of adjacent I signals and the product ofadjacent Q signals.
 7. The device of claim 6, wherein the first singletype frequency discriminator comprises: a first delay unit for delayingan inputted I signal for a certain amount of time; a first multiplierfor multiplying the I signal delayed by the first delay unit and aninputted Q signal; a second delay unit for delaying the inputted Qsignal for the certain amount of time; a second multiplier formultiplying the Q signal delayed by the second delay unit and theinputted I signal; and a first adder for calculating a difference valuebetween outputs of the first and second multipliers and outputting thereal part peak value.
 8. The device of claim 7, wherein the certain timeis a symbol period.
 9. The device of claim 7, wherein the second singletype frequency discriminator comprises: a third multiplier formultiplying an inputted I signal and an I signal which has been delayedfor a certain time; a fourth multiplier for multiplying an inputted Qsignal and an Q signal which has been delayed for the certain time; anda second adder for calculating outputs of the first and secondmultipliers and outputting an imaginary part peak value.
 10. The deviceof claim 9, wherein the certain time is a symbol period.
 11. The deviceof claim 1, wherein the error detecting unit detects an error symbolusing a decision region of QPSK modulation.
 12. The device of claim 11,wherein if a received symbol belongs to a region other than the decisionregion, the error detecting unit determines that the received symbol isan error symbol.
 13. The device of claim 12, wherein the region otherthan the decision region includes regions of${{\tan\left( \frac{r_{c}(t)}{r_{s}(t)} \right)} > {\frac{\pi}{4}\quad{and}\quad{\tan\left( \frac{r_{c}(t)}{r_{s}(t)} \right)}} < {- \left( \frac{\pi}{4} \right)}},$and wherein r_(c)(t) indicates the real part peak value and r_(s)(t)indicates the imaginary part peak value.
 14. The device of claim 11,wherein if a ratio of the real part peak value to the imaginary partpeak value outputted from the frequency discriminating unit is smallerthan −1 or greater than 1, the error detecting unit determines that asymbol used in calculating the real part peak value and the imaginarypart peak value is an error symbol, and outputs a control signal forcutting off the real part peak value and the imaginary part peak valueto the switching unit.
 15. The device of claim 14, wherein if the ratioof the real part peak value to the imaginary part peak value outputtedfrom the frequency discriminating unit is not smaller than −1 but notgreater than 1, the error detecting unit determines that a symbol usedin calculating the real part peak value and the imaginary part peakvalue is a normal symbol, and outputs a control signal for bypassing thereal part peak value and the imaginary part peak value to the switchingunit.
 16. The device of claim 15, wherein the switching unit selectivelybypasses the real part peak value and the imaginary part peak valueoutputted from the frequency discriminating unit under the control ofthe error detecting unit.
 17. The device of claim 1, wherein thefrequency difference average calculating unit adds the real part peakvalue and the imaginary part peak value that have been bypassed by theswitching unit to a previously real part peak value and a previousimaginary peak value by a vector sum method, respectively, andcalculates a real part peak value average and an imaginary part peakvalue average.
 18. The device of claim 1, wherein the received sequenceis a received training sequence.
 19. An AFC (Automatic FrequencyControl) device of a QPSK (Quadrature Phase Shift Keying) modulationsystem comprising: calculating first and second peak values by a symbolperiod using frequency discrimination between adjacent symbols of areceived training sequence; checking whether a ratio of the first peakvalue to the second peak value belongs to a region between first andsecond values; bypassing the first and second peak values if the ratiobelongs to the region; and calculating a frequency difference of thereceived training sequence using the bypassed first and second peakvalues.
 20. The method of claim 19, wherein the first peak value is adifference value between the product of a Q signal of a current symboland an I signal of a previous symbol and the product of an I signal ofthe current symbol and a Q signal of the previous symbol.
 21. The methodof claim 19, wherein the second peak value is a value obtained by addingthe product of an I signal of a current symbol and an I signal of aprevious symbol and the product of a Q signal of the current symbol anda Q signal of the previous symbol.
 22. The method of claim 19, whereinthe first value indicates −1 and the second value indicates +1.
 23. Themethod of claim 19, wherein calculating the frequency differencecomprises: adding the bypassed first and second peak values to theprevious first and second peak values by the vector sum method,respectively, and calculating a first peak value average and a secondpeak value average; and calculating a frequency difference of thereceived training sequence by using an arctan function for a ratio ofthe first peak value average to the second peak value average.
 24. Themethod of claim 19, wherein if the ratio does not come within theregion, the first and second peak values are cut off.
 25. The method ofclaim 19, further comprising: performing AFC using the calculatedfrequency difference.